The present invention relates to a magneto-optical head, magneto-optical device, and magneto-optical recording/reproducing method for use in an external memory device of a computer, etc., and for recording and reproducing of audio and video signals.
Actual application of magneto-optical disks, which are one type of magneto- optical recording medium, has been realized as an external memory device of a computer.
The recording density of the magneto-optical disk is limited by the size of a light beam spot on the magneto-optical disk. That is, as the diameter and interval of recording marks become smaller than the size of the light beam spot, the light beam spot would contain plural bits, making it impossible to separately reproduce each recording bit.
The reproducing resolution of a signal is essentially determined by the wavelength xcex of a light source of a reproducing optical system and by the numerical aperture NA of an objective lens, and the spatial frequency 2NA/xcex sets a limit of reproduction. Thus, to increase recording density, one can take a measure of reducing the light spot diameter of the reproducing device by making the wavelength xcex of the light source shorter and/or by using a high NA lens.
As such, in recent years, to increase recording density of the magneto-optical disk, research has been made to actually reduce the spot diameter of the reproducing device by reducing the wavelength of the laser light used for recording and reproducing, and/or by using a high NA lens. For example, to reduce the wavelength of a laser light, there is research on a semiconductor blue laser, or research in which the wavelength of a laser light is reduced from around 800 nm to 400 nm with the use of a second harmonic wave generating element (SHG). These research has not reached the stage which can be brought into actual application due to their stability, performance, and cost. However, once realized, it is certain that they will bring higher density recording of information than that offered by the current optical disk systems.
However, the wavelength of a semiconductor laser as currently available in actual application is only around 650 nm.
Further, when a high NA lens is used, the depth of focus is reduced, and high precision is required in the distance between the lens and disk, and it becomes difficult to manufacture the optical disk with precision. For this reason, the NA of the lens cannot be increased substantially, and the lens NA which can be brought into actual application is only around 0.6. Thus, there is a limit in reducing the wavelength of the light source or increasing the NA of the lens, and as it currently stands, it is difficult to effectively increase recording density by these measures.
In view of these drawbacks, for example, Journal of The Magnetics Society of Japan, Vol. 19, Supplement, No. S1 (1995), pp. 421-424 (Document 1) recites a magnetically induced super resolution (xe2x80x9cMSRxe2x80x9d hereinafter) technique, which is the method for increasing recording density by improving the reproducing resolution with the use of a magneto-optical recording medium which is composed of magnetostatically coupled two magnetic layers and by utilizing a temperature distribution of the light beam spot.
Also, for example, Applied Physics Letter No. 69 (27), Dec. 30, 1996, pp. 4257-4259 (Document 2) recites that information is reproduced while applying an alternating magnetic field to an MSR medium employing magneto-static coupling so that a recording mark is enhanced when transferring the recording mark of the recording layer to the reproducing layer, thus increasing the amplitude of the reproduced signal.
As another method of increasing recording density without reducing the laser wavelength, the numerical aperture (NA) of the optical system is increased. For example, Applied Physics Letter, No. 68(2), Jan. 8, 1996, pp. 141-143 (Document 3) discloses a technique in which an effective NA is increased with the use of a solid immersion lens (SIL) so as to reduce the beam spot.
The following will describe Document 1 in more detail referring to FIG. 13 and FIG. 14.
FIG. 13 shows a representative arrangement of a conventional MSR magneto-optical recording medium. On a transparent substrate 61, there are deposited a transparent dielectric layer 62, reproducing layer 63, transparent dielectric layer 64, recording layer 65, and transparent dielectric layer 66.
The recording layer 65 records magneto-optical information in the form of a change in length of recording marks. However, FIG. 13 and FIG. 14 illustrate an example of only the shortest recording mark. This is because reproduction of the shortest recording mark would automatically allow reproduction of signals of longer recording marks. Thus, it is assumed that the lengths of recording marks are the same, and a region in which the shortest recording mark is to be formed is schematically divided into domains. In the following, each domain will be referred to as a magnetic domain.
Each of magnetic domains A through I records a signal as shown in FIG. 13 and FIG. 14. Even though the reproducing layer 63.is not divided into a plurality of magnetic domains unlike the recording layer 65, for convenience of explanation, (refer to FIGS. 4(a) and 4(b) and FIG. 14), domains of reproducing layer 63 corresponding in position to the magnetic domains A through I of the recording layer 65 are indicated by domains Axe2x80x2 through Ixe2x80x2.
The following considers the case where laser light is projected on the center of magnetic domain E of the recording layer 65, spreading over the region larger than the magnetic domain E. The temperature distribution of the recording layer 65 would then take the shape in which a high temperature portion (e.g., 150xc2x0 C.) is found at the site of the recording mark E and the temperature decreases as it moves further from the magnetic domain E. Further, the size of saturation magnetization also takes the distribution which reflects the temperature distribution of the recording layer 65 and it becomes maximum in the magnetic domain E.
Meanwhile, the reproducing layer 63 has in-plane magnetization parallel to the plane of the film (perpendicular to the plane of the paper) at room temperature, and it has a xe2x80x9cmask regionxe2x80x9d from which no signal is reproduced.
By being heated to a high temperature by projection of a laser beam, the magnetization of the reproducing layer 63 becomes smaller and the reproducing layer 63 comes to have perpendicular magnetization, forming an xe2x80x9caperture regionxe2x80x9d to which the magnetization of the recording layer 65 is transferred by a magneto-static force.
In reproduction, a temperature distribution is generated in the laser spot, and a signal is reproduced only from the aperture region formed at the high temperature portion of the temperature distribution. Namely, by the magnetic flux generated from the magnetic domain E, adjoining portion Exe2x80x2 of the reproducing layer 63 is subjected to a force (magneto-static force) which is in accordance with the saturation magnetization of the magnetic domain E, making saturation magnetization of magnetic domains E and Exe2x80x2 in line. In this manner, in the transfer of a recording mark of the recording layer 65 to the reproducing layer 63, transfer of a signal to the reproducing layer 63 occurs only at the magnetic domain E, and the recording marks of the other magnetic domains (A through D and F through I) are not transferred and remain as a mask region, thus limiting the signal reproducing region and effectively reducing the reproduced spot.
Thus, even when the recording mark is smaller than the beam spot diameter, information can be read out without interfering with recording marks of adjacent magnetic domains, thus increasing the reproducing resolution of signals and realizing high density recording. Further, because the adjacent tracks at room temperature make up a mask region, a signal leak (cross talk) from adjacent tracks hardly occurs. As a result, the intervals between recording tracks can be reduced.
FIG. 14 describes how this is done in more detail. As the material of the reproducing layer 63 and recording layer 65, an alloy of rare earth metal and transition metal (RE-TM) is used. It is assumed here that an x-y coordinate system is found within a plane parallel to the recording layer 63 and reproducing layer 65, in which the direction of TM magnetization in the plane of the reproducing layer 63 represents y axis, and the direction orthogonal to y axis represents x axis, and the direction which is orthogonal to the both axes x and y and which is in the direction of layer deposition represents z axis. In the recording layer 65, the directions of TM magnetization and saturation magnetization are parallel to z axis, and due to the fact that the composition of the recording layer 65 is TM rich (sub-lattice magnetic moment of the transition metal at room temperature exceeds sub-lattice moment of the rare earth metal), TM magnetization and saturation magnetization direct in the same direction. On the other hand, at a low temperature portion of the reproducing layer 63, the directions of TM magnetization and saturation magnetization are found within the x-y plane, and by the fact that the composition of the reproducing layer 63 is RE rich (sub-lattice magnetic moment of the rare earth metal at room temperature exceeds sub-lattice moment of the transition metal), the saturation magnetization and TM magnetization direct in the opposite directions.
When a laser spot is projected on magnetic domain E in reproduction, by the magnetic flux generated from the magnetic domain E, the saturation magnetization of Exe2x80x2 of the reproducing layer 63, which has risen to a high temperature becomes in accordance with the saturation magnetization of magnetic domain E. The site of the reproducing layer 63 other than Exe2x80x2 is at a low temperature and the magnetization therein remains in in-plane direction (x-y plane).
Incidentally, in magnetic substances, when magnetization is in close proximity, by the exchange interaction, there arises a force (exchange force) which acts to align the TM magnetization in the same direction. In the reproducing layer 63, each TM magnetization of Axe2x80x2 to Ixe2x80x2 is exchange-coupled with adjacent TM magnetization, and the TM magnetization of Exe2x80x2 is exchange-coupled with the TM magnetization of adjacent Dxe2x80x2 and Fxe2x80x2. Namely, the TM magnetization of Exe2x80x2 is subject to a force of TM magnetization of Dxe2x80x2 and Fxe2x80x2, which force acts to direct the direction of TM magnetization of Exe2x80x2 in an in-plane direction. Inversely, the TM magnetization of Dxe2x80x2 and Fxe2x80x2 are subject to a force of the TM magnetization of Exe2x80x2, which force acts to direct the direction of the TM magnetization of Dxe2x80x2 and Fxe2x80x2 in a perpendicular direction. However, the TM magnetization of Dxe2x80x2 and Fxe2x80x2 are subject to a force respectively from Cxe2x80x2 and Gxe2x80x2, which are adjacent to Dxe2x80x2 and Fxe2x80x2 respectively on the other sides of Exe2x80x2, which force acts to direct the TM magnetization of Dxe2x80x2 and Fxe2x80x2 in an in-plane direction, and because the force is large, the TM magnetization of Dxe2x80x2 and Fxe2x80x2 are stably directed in an in-plane direction.
Thus, transfer of a recording mark from the recording layer 65 to reproducing layer 63 occurs at a site where the magneto-static force between the recording layer 65 and reproducing layer 63 exceeds the exchange force within the reproducing layer 63. That is, in this case, the exchange force existing within the reproducing layer 63 acts to reduce the transferred recording mark.
FIG. 15 shows recording mark length dependency of a reproduced signal amplitude when the conventional MSR magneto-optical disk as described above is reproduced by an optical system having a light spot diameter of 0.9 xcexcm. For comparison, FIG. 15 also shows the result of reproduction with the use of a non-MSR conventional magneto-optical disk. Since reproducing resolution is improved in the MSR magneto-optical disk, a larger reproduced signal amplitude is found with a smaller recording mark as compared with the conventional magneto-optical disk.
However, the prior arts have the following problems.
In the MSR technique as recited in Document 1, to improve reproducing resolution, a mask region is provided over the reproducing layer, and the transferred recording mark is reduced as it is subjected to a force of the mask region which acts to reduce the recording mark. When a small recording mark is reproduced, a reproduced signal amplitude is also reduced, and a sufficient signal amplitude cannot be obtained. This essentially limited the smallest recording mark which could be read out when reproducing the conventional MSR magneto-optical disk, and this limitation prevented improvement of recording density.
Further, when the reproduced signal amplitude is to be made larger, an additional energy, etc., such as application of an alternating magnetic field is required in a reproducing operation as in Document 2, which resulted in increased power consumption.
Further, in the method using SIL as in Document 3, while it is possible to reduce the beam sot diameter, reproducing resolution which exceeds the beam spot diameter cannot be obtained, and even when SIL and MSR technique was combined, the problems still remain that the reproduced signal amplitude is reduced when recording mark is reduced, and that it requires an additional energy to increase the reproduced signal amplitude.
It is an object of the present invention to provide a magneto-optical head which is capable of increasing recording density by means of preventing a reproduced signal amplitude from being reduced when a recording mark of a magneto-optical recording medium is reduced.
The present invention was accomplished after extensive research by finding that with the use of an SIL which includes an auxiliary lens which is provided with a magnetic layer for transferring information written on the medium by enhancing the information, a recording mark which is even smaller than a beam spot diameter, which was reduced smaller than SIL, can be reproduced without applying a reproducing magnetic field and without reducing the reproduced signal amplitude.
In order to achieve the foregoing object, a magneto-optical head of the present invention for carrying out recording and reproducing on a recording medium having a recording layer which has a maximum value of saturation magnetization between room temperature and a Curie temperature includes: an objective lens for converging light for raising a temperature of the recording layer; an auxiliary lens for projecting the converged light on the recording layer by increasing an effective numerical aperture; and a reproducing magnetic layer, provided on the auxiliary lens, for reproducing information recorded on the recording layer by temporarily enhancing and transferring the information, and the reproducing magnetic layer is provided on a position on which the converged light is projected.
With this arrangement, by setting a spacing between the auxiliary lens and recording layer smaller than a wavelength of light, the light spot which was converged by the objective lens and which was increased in effective numerical aperture by the auxiliary lens is transferred to the recording layer by the near field effect as set above, i.e., projected on the recording layer. Here, compared with the case where the light is converged only by the objective lens, light can be projected on even a smaller area.
Projection of the light causes the recording layer to generate a temperature distribution. As a result, at a portion of the recording layer which exceeds a certain temperature, saturation magnetization becomes larger, and there generates magnetic flux in accordance with pre-recorded information. Here, such a temperature raised portion can be made smaller than a spot diameter of the light with the use of the light of appropriate power. Thus, there generates magnetic flux from saturation magnetization which is directed in accordance with the recorded information over the region of the recording layer smaller than the spot diameter of the light. Therefore, by setting the temperature raised portion to match the area of the recording layer on which the smallest unit of information is recorded (xe2x80x9crecording markxe2x80x9d hereinafter), high density magnetic flux which is in accordance with the information from the information of each recording mark can be generated to reach the reproducing magnetic layer.
In the reproducing magnetic layer having an appropriate area, the directions of saturation magnetization are aligned over the entire reproducing magnetic layer in accordance with the magnetic flux. Thus, by increasing the area of the reproducing magnetic layer larger than the area of the recording mark, the information recorded on the recording mark can be transferred to the reproducing magnetic layer by being enhanced.
Meanwhile, by the reproducing magnetic layer which is provided on the position of the light spot, there generates reflected light by projection of the light. The reflected light is rotated in the direction of the polarization direction with respect to incident light by the effect of the directions of the saturation magnetization of the reproducing magnetic layer, and a reproduced signal is generated based on this rotation in the polarization direction. Here, because the magnetization transferred on the reproducing magnetic layer is enhanced, the quantity of the light which was rotated in polarization direction (Kerr rotation), i.e., which contains information, becomes larger than the case in which reflected light is utilized directly from the recording mark, thus increasing a reproduced signal amplitude.
In this manner, the reproduced signal amplitude is prevented from being reduced even when the recording mark is made smaller, thus making it possible to increase recording density while maintaining a large reproduced signal amplitude.
As a result, with the described arrangement, it is possible to read out a recording medium which records information in high density by the reduced recording mark, thus realizing higher density recording of information than the conventional art.
A magneto-optical device of the present invention includes the magneto-optical head as described above.
With the above arrangement, information can be recorded and reproduced in high density with respect to a recording medium, which allows the use of a higher density recording medium than the conventional medium, thus increasing the amount of information per recording medium to be inserted in the device. As a result, it is possible to reduce the size of the recording medium and the magneto-optical device to which the recording medium is to be inserted, and the number of recording medium used can be reduced as well.
Further, with the described arrangement, high density recording of information is realized without requiring application of an external magnetic field, and thus compared with the conventional magneto-optical device which realizes high density recording and reproducing by the method accompanying application of an external magnetic field in reproduction, power consumption can be reduced.
The method of the present invention for magneto-optically recording and reproducing information using a magneto-optical head for carrying out recording and reproducing on a recording medium having a recording layer which has a maximum value of saturation magnetization between room temperature and a Curie temperature includes: an objective lens for converging light for raising a temperature of the recording layer; an auxiliary lens for projecting the converged light on the recording layer by increasing an effective numerical aperture; and a reproducing magnetic layer, provided on the auxiliary lens, for reproducing information recorded on the recording layer by temporarily enhancing and transferring the information, the reproducing magnetic layer being provided on a position on which the converged light is projected, wherein, in reproducing, light is projected on the recording layer via the reproducing magnetic layer provided on the auxiliary lens, and information recorded on the recording layer is transferred to the reproducing magnetic layer by magnetic flux generated from the recording layer so as to reproduce the information using reflected light from the reproducing magnetic layer, and in recording, light is projected on the recording layer by varying an energy of the light from that used in reproducing, and an external magnetic field based on information to be recorded is applied so as to record the information on the recording layer.
With this method, by varying the light energy between reproducing and recording, specifically, by projecting the light of higher energy in recording than in reproducing, the recording layer of the recording medium is heated to a temperature in the vicinity of the Curie temperature, and by applying an external magnetic field to a portion where the coercive force has decreased, information can be recorded in high density using a single magneto-optical head. As a result, it is possible to provide a magneto-optical device having a simple structure, which can be easily switched between recording and reproducing operations.
In order to achieve the foregoing object, the magneto-optical head of the present invention includes: an objective lens for converging light on a recording layer of a recording medium; an auxiliary lens for allowing the light to travel toward the recording layer by a near field effect by increasing a numerical aperture of the objective lens; and a reproducing magnetic layer for reproducing information of a recording mark by enhancing and transferring magnetic flux which was generated only from a single recording mark of the recording layer by heat of the light fallen on the recording layer, and the reproducing magnetic layer is provided on a side of the auxiliary lens toward which the light travels.
With this arrangement, provided that the recording layer recording information as magnetization within the recording medium is positioned sufficiently close to the transparent element, when the light incident from the side of the lens element is converged on the reproducing magnetic layer in the vicinity of the surface of the transparent element on the side of the recording medium, the light is also transferred to the recording layer by the near field effect. The recording layer, having received the light, is heated and comes to have saturation magnetization in accordance with recorded magnetization.
Here, when the light energy is appropriate, the recording layer can be heated only in the vicinity of a portion of a single recording mark of the recording layer, thus generating strong saturation magnetization of a single recording mark. The magnetic flux generated from the saturation magnetization is enhanced and transferred to the reproducing magnetic layer having a wider area than that of the recording mark. The reproducing magnetic layer is irradiated by the light, and generates a reproduced signal using the reflected light which was subjected to the effect of enhanced and transferred magnetization.
Because the magnetization contributing to the reproduced signal is enhanced from the magnetization of the recording medium, the reproduced signal amplitude can be made larger. That is, even when the magnetization recorded on the recording medium is made smaller, the reproduced signal amplitude can be prevented from being reduced, thus realizing high density recording of information on the recording medium.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.